1. Field of the Invention
This invention relates to a three-phase three-level inverter for use in an electric railcar or the like.
2. Description of the Related Art
Conventionally, a type of three-phase two-level inverter such as that shown in FIG. 10 has been used for inverter electric railcars. This inverter operates in a constant-V/f control mode called a three-pulse mode in the range of inverter frequencies lower than a predetermined frequency. This mode is based on a synchronous pulse width modulation method in which sinusoidal waves and a triangular wave are compared and, in this mode, the frequency of the triangular wave is three times as those of the sinusoidal waves. In the range of frequencies higher than the predetermined frequency, the inverter operates in a whole voltage control mode called a one-pulse mode.
Referring to FIG. 10, the inverter has a DC power source 1, a reactor 2 connected to a plus terminal of the DC power source 1, and a capacitor 3 connected between the reactor 2 and a minus terminal of the DC power source 1. Three pairs of switching elements 4-5, 6-7, and 8-9 each connected in series are connected in parallel with the capacitor 3. Free-wheeling diodes 10 to 15 are connected in parallel with the respective switching elements 4 to 9 with reverse polarities. A phase-U output terminal 16, a phase-V output terminal 17 and a phase-W output terminal 18 extend from middle points between the pairs of switching elements.
The reactor 2 and the capacitor 3 form a DC filter, and each of the switching elements 4 to 9 is a GTO thyristor, a power transistor or the like.
The operation of this inverter will be described below with reference to FIGS. 11 and 12. Referring to FIG. 11, in the three-pulse mode, a phase-U output voltage V.sub.u is obtained from a phase-U sinusoidal modulating wave S.sub.u and a triangular carrier wave S.sub.c, and a phase-V output voltage V.sub.v is obtained from a phase-V sinusoidal modulating wave S.sub.v and the triangular carrier wave S.sub.c. The line U-V voltage V.sub.v is obtained by EQU V.sub.uv =V.sub.u -V.sub.v
In the one-pulse mode, the line U-V voltage V.sub.uv1 is exhibited as a rectangular wave having a width of 120.degree..
In ordinary operation of conventional inverter electric railcars, the operation is changed over from the three-pulse mode to the one-pulse mode as the inverter frequency is increased; the inverter operates in the one-pulse mode in the range of high inverter frequencies. However, at the time of transition from the three-pulse mode to the one-pulse mode, a voltage jump of about 10 % of the whole voltage takes place as shown in FIG. 12. This phenomenon is described, for example, on page 26 of Hitachi Hyouron (Hitachi Review), Vol. 68, No. 8 (1986-8).
In the three-pulse mode, the ratio of the amplitude of each phase sinusoidal modulating wave to the amplitude of the triangular carrier wave S.sub.c, i.e., the percentage modulation is changed to control the angle .theta. which is determined by the points of intersection of each phase sinusoidal modulating wave and the triangular carrier wave S.sub.c as shown in FIG. 11, thereby changing the line U-V voltage V.sub.uv. However, the angle .theta. has a minimum value .theta..sub.min &gt;0 depending upon restrictions owing to the minimum switching element off time, setting of a period of time for preventing short-circuit of upper and lower arms of the inverter unit, and so on.
In contrast, the line voltage V.sub.UV1 in the one-pulse mode forms a 120.degree. width rectangular wave, as mentioned above. At the time of transition from the three-pulse mode to the one-pulse mode, therefore, the line voltage is abruptly increased by a step corresponding to the width .theta. slit. That is, referring to FIG. 12, when the inverter operates for change-over from the three-pulse mode to the one-pulse mode at the inverter frequency f.sub.0, change-over is actually effected at an inverter frequency f.sub.1. In FIG. 12, the dot-dash line indicates the change in the line voltage in an ideal state.
In the conventional inverter, as described above, a discontinuous change in the line voltage of about 10 % occurs at the time of transition from three-pulse mode operation to one-phase mode operation, resulting in occurrence of an undesirable phenomenon such as an abrupt change in the inverter output current or output torque or voltage oscillation of the DC filter.